1. Field of the Invention
The present invention relates to an image forming apparatus and a method for controlling image density in the image forming apparatus, and more particularly to an image forming apparatus and a method for controlling image density in the image forming apparatus.
2. Discussion of the Background
Background image forming apparatuses, such as printers, facsimiles, copiers, and multifunction apparatuses which print, fax, copy, and so on, generally use an electrophotographic process for image formation. Such image forming apparatuses need to print high-quality pictures and operate reliably. More specifically, such image forming apparatuses need to maintain good image quality unaffected by environmental changes and provide consistently high-quality images.
To satisfying such requirements, a two-component developer development method has become widely known in recent years because it is easy to apply to color image formation.
In the two-component developer development method, two-component developer (hereinafter simply “developer”) that includes both non-magnetic toner and magnetic carrier is used. The two-component developer is held on a developer carrier (hereinafter “sleeve”). A magnetic brush is formed by magnetic poles provided inside the sleeve. A development bias is applied at a position where the development sleeve faces a latent image carrier (hereinafter “photoreceptor”) to develop an electrostatic latent image on the photoreceptor into a visible toner image. Rotation of the development sleeve brings the two-component developer to a development zone, where a large amount of magnetic carrier covered with toner in the developer rolls up along magnetic field lines to form the magnetic brush.
Unlike a single-component developer development method, with the two-component developer development method it is extremely important to control weight ratio (toner concentration) between toner and carrier precisely to improve stable operation. For example, when the toner concentration is too high, the image may appear grainy, with decreased detail resolution. When the toner concentration is too low, density in filled-in areas is decreased, and a carrier adhesion problem may occur. To control image density appropriately, it is necessary to control toner concentration in the developer by controlling the amounts in which the toner is supplied, so that the toner concentration is kept within a predetermined range.
Toner concentration control can be performed using a toner concentration detector (permeability sensor). By comparing an output value Vt of the toner concentration detector (permeability sensor) with a toner concentration control reference value Vtref, a proper toner supply amount is then determined based on the comparison result. A permeability sensor is generally used to detect the toner concentration, in which current toner concentration is detected based on a change in permeability of the developer due to change in toner concentration, obtained by comparing detected permeability with a reference value of the toner concentration.
Alternatively, an optical sensor may also be used for detecting the toner concentration. In a method using the optical sensor, a reference pattern is formed on an image carrier or an intermediate transfer belt. The optical sensor detects reflected light from the reference pattern to obtain any difference between an image portion and a non-image portion of the reference pattern. The toner concentration is obtained based on the detected difference.
The reference pattern is typically formed in an interval between successive printing processes, to successively provide the toner concentration control reference value Vtref. However, there is strong demand to reduce consumption of the toner used to form the reference pattern between sheets of recording media. Accordingly, this method has not been employed in most apparatuses recently. Further, when the reference pattern is formed on the intermediate transfer belt, it is necessary to employ a cleaning device on a secondary transfer roller. Accordingly, it may be preferable to avoid use of this method in which the reference pattern is formed between successive sheets of paper or the like from the point of view of mechanical cost reduction. Therefore, it is all the more important to control the toner concentration using a single permeability sensor during successive printing operations and imaging mode change (change in linear process velocity).
In the permeability detection method, the permeability of magnetic carrier is detected to get the permeability of the developer. Accordingly, if a bulk density of the developer is changed, the permeability of the developer is also changed, resulting in a change in the detection output value. More specifically, even if the ratio of toner to carrier remains constant, the permeability of the developer is changed because the carrier amount per unit volume in developer is changed when the bulk density of the developer is changed. As a result, the output value of the toner concentration sensor may be changed.
For example, in an image forming system which has image output modes covering a plurality of different linear velocities, a rotation speed of an agitation screw provided in the development unit varies in the image output mode. More specifically, even if the toner concentration remains the same, the output value of the toner concentration sensor is varied by the bulk density, charging amount and flowability of the developer due to a change of the agitation speed when the process linear velocity is changed. Hereinafter, a difference of the output value Vt of the toner concentration sensor corresponding to a difference of process linear velocity is expressed as a linear velocity shift.
When the output value of the toner concentration sensor is changed even with the same toner concentration at the change of the process linear velocity, it is not possible to control the toner concentration of the development unit to keep a predetermined toner concentration. Therefore, the linear velocity shift at the change of the linear velocity is obtained preliminarily from experimental data to use as the toner supply amount.
JP-2002-207357-A describes an image forming apparatus that includes a permeability sensor that detects the toner concentration of the developer in the development unit. The detected value is compared to a threshold value, and based on the comparison result, the toner concentration of the developer in the development unit is controlled. Further, the threshold value for the detected value of the toner concentration is changed in accordance with the change of the linear velocity in the image forming apparatus.
JP-2007-71985-A describes an image forming apparatus which performs an adjustment mode to detect the linear velocity shift. When certain predetermined conditions are satisfied, the image forming apparatus performs the adjustment mode so that the linear velocity shift is renewed.
However, in both prior arts disclosed in JP-2002-207357 and JP-2007-71985, compensation amount of the linear velocity shift is constant when the toner concentration is changed. In other words, it is possible to correct the output value of the sensor for a certain condition of the toner concentration. However, in a development unit in which the toner concentration is changed due to a change of the linear velocity shift, some errors may be observed in the compensation amount.